Field of the Invention
The invention relates to a device for monitoring the current, supplied to a load from an alternating-current and particularly three-phase network via a controlled rectifier, comprising measurement transformers for detecting the current, a signal generator for generating a reference value for the current and a comparator which supplies a switching-off signal when the current to be monitored exceeds the predetermined reference value of the signal generator.
Discussion of Prior Art
Such known devices (German Offenlegungsschrift No. 3,032,757) are used, for example, in electro-chemical machining (i.e. ECM). The current supplied by the rectifier to the resistive to resistive-inductive load can have a varying ripple percentage with the same arithmetic mean value, depending on operating parameters (for example operating voltage, operating gap size). However, since the arithmetic mean is determining for the loading of the electrode and the result of the work, it is important to take this arithmetic mean into consideration when switching off. But the calculation of the arithmetic mean by integrating the instantaneous values entails a delay until switching-off is possible, which delay can have a negative effect with regard to the risk of damaging the electrode and the quality of machining. For this reason, the maximum values of the operating current suitable for a permissible arithmetic mean of the current have hitherto been determined experimentally for certain operating parameters. But if the operating parameters change, these maximum values determined no longer correspond to the arithmetic mean value of the current which would be optimum for these changed operating parameters.